The use of 4,4'-methylene bis (2-chloroaniline) known as "MOCA" (as described in the U.S. Pat. No. 3,188,302) as a curing agent for polyurethanes is well-known and widespread in industry, but unfortunately this compound has been found to possess certain disadvantageous features. Propbably its most serious disadvantage is its apparent carcinogenicity which has resulted in regulations by the Occupational Safety and Health Administration strictly controlling its use in polyurethane processing. The regulations are such as to effectively preclude the continued use of MOCA in many commercial polyurethane processes. A replacement for MOCA is thus necessary.
Previous attempts to replace MOCA in commercial polyurethane processing have not been commercially successful and have presented other problems. In U.S. Pat. No. 3,629,168, for example, there is disclosed a curing agent which is purported to be useful as a replacement for MOCA, which curing agent is a lower alkylene-bis (2---carboxyaniline) such as methylene bis (methyl anthranilate) (MBMA). MBMA, however, and similar compounds have a relatively high melting point (e.g., above 275.degree. F.) and when molten MBMA is mixed with a urethane pro-polymer at conventional curing temperatures (i.e., about 212.degree. F.), the urethane polymerization reaction is very difficult to control. Also, introduction of a relatively high melting point (e.g., 250.degree. or more .degree. F.) compound into a prepolymer melt which has a temperature of about 212.degree. F. generally results in the crystallization of the higher melting point compound. Raising the prepolymer melt temperature in the range of about 250.degree. F. minimizes the crystallization problem but the pot life of the prepolymer is so short (about 2 minutes with most conventional urethane prepolymers) that there is insufficient time for mixing and pouring of the prepolymer. In order to control the urethane polymerization reaction using these relatively high melting point compounds at conventional curing temperatures, it is necessary to utilize the high melting point compound such as MBMA as a dispersed solid or dispersed liquid in an appropriate liquid diluent. Both types of dispersions, however, result in an unsatisfactory process and end product. Whent dispersed into the prepolymer, the MBMA particles do not dissolve. The particles remain intact and react only around the outside of the particle so that only a small portion of the MBMA reacts to cure the prepolymer. The balance of the uncured prepolymer slowly cures on exposure to air due to the reaction of the available isocyanate groups in the prepolymer with moisture in the air. The resulting products are characterized by their poor physical properties, particularly their low tear and tensile strengths and hardness value. A urethane polymerization process utilizing these types of solutions results in very large amounts of unreacted materials thus decreasing process efficiency and increasing product recovery problems.
U.S. Pat. No. 3,681,291 suggests replacing MOCA with the reaction product of (a) a polyoxypropylene glycol or polytetramethylene ether diol having a molecular weight of from about 500 to about 3000; (b) an aromatic diol; and a particularly defined aromatic amine, in order to provide curing an ambient temperatures. The resulting polyurethane product, however, has very low tensile strengths (about 2000 psi maximum) and requires an organometallic catalyst for complete reaction.
U.S. Pat. No. 3,728,310 discloses a curing agent for isocyanate-terminated polyurethanes prepared by the acid-catalyzed reaction of 3-chloroaniline, 2,5-dichloroaniline and formaldehyde which curing agent is alleged to be a suitable substitute for MOCA. Other diamine curing agents, however, are formed from one or more diamine compounds the use of which also have been severely restricted by OSHA.